(1) Field of the Invention
The present invention relates to semiconductor laser elements and devices using the same to optically record or reproduce data. More particularly, the invention relates to an invention useful for nitride semiconductor laser elements.
(2) Description of the Prior Art
Conventionally, studies have been conducted on semiconductor laser elements comprising various semiconductor materials. Semiconductor laser elements now in practical use are mainly those that radiate laser beams of relatively long wavelengths, from red spectrum to green spectrum (e.g., arsenide semiconductor laser elements comprising an arsenide semiconductor material used for the active layer). Such semiconductor laser elements are mainly used as light sources of devices to optically record or reproduce data, or of devices to optically transmit data.
In optical data recording devices, a need for speed enhancement of data recording has led to a growing demand for semiconductor lasers generating stable laser beams at a high output power level. Further, in such recording media, a need for more highly dense data recording has led to a growing demand for semiconductor laser elements that radiate laser beams with short wavelengths.
In common optical data recording devices, a recording manner is one which harnesses the crystal phase change or magnetic phase transition of a material composing the recording layer of a recording medium. Here, energy of the laser beams is used as a heat source in recording data. Thus, high-speed data recording involved a high output power of the laser beams.
However, when the amount of current injected is increased to enhance the output power of the laser beams, semiconductor laser elements are known to oscillate in a high-order horizontal-lateral mode as well as in the basic horizontal-lateral mode. The strength of laser beams oscillating in a high-order horizontal-lateral mode distributes in the spot differently from the strength of laser beams oscillating in the basic horizontal-lateral mode. This can increase the diameter of the spot and change the ellipticity thereof to a value away from 1. In addition, oscillations in different horizontal-lateral modes make it difficult to radiate stable laser beams. As a result, it can be that laser beams are not narrowly focused, the spot of converged laser beams is easily dislocated, and recording density is lessened.
The lateral mode includes the horizontal-lateral mode and the perpendicular-lateral mode, and the horizontal-lateral mode can be problematic when oscillations in a high-order horizontal-lateral mode occur. This is because, in a manufacturing process, length is more accurately processed in the direction of deposition of the semiconductor layers composing the laser structure than in the direction perpendicular to the deposition direction. Thus, oscillations realized are only in the basic perpendicular-lateral mode rather than in a high-order perpendicular-lateral mode. Such element structure is enabled.
Common semiconductor laser elements to radiate laser beams of short wavelengths include nitride semiconductor laser elements to radiate laser beams from blue spectrum to ultraviolet spectrum. Such elements use, as the active layer, nitride semiconductor materials represented by GaN, InN, AIN, or mixed crystal semiconductors thereof In recent years, such elements have been prototyped. Examples include a nitride semiconductor laser element with a continuous wave operation in room temperature (room temperature CW) in the basic horizontal-lateral mode until output power reaches the order of 30 mW, as described in “Extended Abstracts for the 47th meeting of The Japan Society of Applied Physics and Related Societies, 31p-YQ-8.” Another example is a nitride semiconductor laser element with a continuous wave operation in room temperature (room temperature CW) in the basic horizontal-lateral mode until output power reaches the order of 30 to 40 mW, as described in “Japanese Journal of Applied Physics, Vol. 39, pp. 647–650, 2000,” by Shin-ichi Nagahama et al. The far field pattern (FFP) of this element is that the full angle of half maximum in the perpendicular direction is 25 degree and that in the horizontal direction is 8 degree. Laser beams of short wavelengths can make the spot focused more narrowly (or make the spot diameter narrower) than laser beams of long wavelengths. Thus, attention is paid to optical data recording devices that use nitride semiconductor laser elements as the light sources, in hopes of such devices acting as high density data recording devices of the next generation that might enable high density recording of the recording media, as compared with conventional devices.
Specific techniques to maintain stable oscillations of a semiconductor laser element until output power reaches a high level will now be described below.
Instability due to an asymmetrical distribution of carriers in the horizontal direction and of light intensity in the horizontal direction in the striped region needs to be corrected, and oscillations in a single horizontal-lateral mode (only in the basic horizontal-lateral mode) until output power reaches a high level needs to be realized. For those purposes, AlGaAs semiconductor laser elements are known to employ a light waveguide of what is called an effective refractive index waveguide type (ridge stripe structure). For oscillations to maintain up to a high output power level in the basic horizontal-lateral mode, it is effective to narrow the light waveguide in width (hereinafter referred to as stripe width). It is known, however, that making stripe width too narrow weakens confinement of light in the basic horizontal-lateral mode, and causes a rise in the threshold voltage of the semiconductor laser element. Thus, stripe width has optimum values.
A technique to optimize stripe width can be applied to nitride semiconductor laser elements. The optimum values of stripe width, however, are approximately proportional to the wavelengths of laser oscillations, and thus become as low as 2 μm with nitride semiconductor laser elements, as compared with AlGaAs semiconductor laser elements. Thus, adverse effects on the characteristics of semiconductor laser elements become distinguished as the value of stripe width is set narrower. To sufficiently reduce adverse effects on the characteristics of nitride semiconductor laser elements, a technique to keep the deviation of stripe width within ±0.1 μm from the set value is required. It is, however, technically very difficult to keep a deviation within the range. Therefore, in nitride semiconductor laser elements, oscillations in a single horizontal-lateral mode (only in the basic horizontal-lateral mode) until output power reaches a high level are difficult to realize only by stripe width adjustment.
FIG. 11 is a schematic section of the structure of a conventional semiconductor laser element. The direction perpendicular to the drawing is that of the resonator (axis direction). This semiconductor laser element has a substrate 1101, a lower clad layer 1103, a lower light waveguide layer 1104, an active layer 1105 having a triple quantum well structure, a carrier block layer 1106, an upper light waveguide layer 1107, an upper clad layer 1108, and a contact layer 1109 deposited in this order. The upper clad layer 1108 has on its center a striped protruding portion whose sectional shape is convex. The contact layer 1109 is only formed on approximately the entire surface of the protruding portion of the upper clad layer 1108. The protruding portion of the upper clad layer 1108 and the contact layer 1109 consist the ridge-stripe structure of this semiconductor laser element. The ridge-stripe structure is formed by using photolithography and etching techniques after forming an upper clad film to be formed into the upper clad layer 110 and a contact film to be formed into the contact layer 1109. The etched regions of the contact layer are completely removed, and the etched regions of the upper clad layer are left with a predetermined thickness. In this semiconductor laser element, light emitted from the active layer 1105 is confined in the waveguide structure, where then there are laser oscillations.
This semiconductor laser element further has a buried layer 1110 that is formed in the approximately entire etched regions and is approximately transparent for light with the oscillation wavelength, an electrode 1112 formed over the approximately entire surfaces of the contact layer 1109 and the buried layer 1110, and an electrode 1111 formed on the approximately entire surface of two main surfaces of the substrate 1101. The surface with the electrode 1111 is opposite to the other surface on which the lower clad layer 1103 is formed. The both end surfaces of the ridge-stripe structure are those of the resonator and serve as the mirrors to the resonator.
FIG. 12 is a conceptual section of the structure of a conventional semiconductor laser element. This semiconductor laser element is similar to the semiconductor laser element of FIG. 11, except that the substrate used is a sapphire substrate 1201, and that a negative electrode 1211 is formed in the region that results from photolithography and etching and contacts the lower clad layer 1203.
In a semiconductor laser element having a structure such as one shown in FIG. 11, known methods to improve oscillations through up to a high output power level in a single horizontal-lateral mode (only in the basic horizontal-lateral mode) include adjustment of the total thickness (residual thickness after etching) of the upper clad layer 1108 outside the ridge-stripe region and the upper light waveguide layer 1107.
An increase in the residual thickness after etching narrows a difference in a local effective index between the regions within and outside the striped region, and lessens the horizontally-directed confinement coefficient of light in the 1st-order horizontal-lateral mode, as compared with the basic horizontal-lateral mode. However, an excessive increase in the residual thickness after etching lessens the horizontally-directed confinement coefficient of light in the basic horizontal-lateral mode, causing a rise in the threshold voltage of the semiconductor laser element. Further, an excessive increase in a residual thickness after etching horizontally widens the intensity distribution of emitted light, and causes the far field pattern (FFP) of the radiated laser beam to be such that the full angle of half maximum in the horizontal direction is narrowed. This causes the radiated laser beam to deteriorate its ellipticity (in the FFP, ratio of the full angle of half maximum in the perpendicular direction to that in the horizontal direction). If such semiconductor laser element is installed in optical data recording/reproducing devices, the connecting efficiency of laser beams to the optical system decreases. Thus, the residual thickness after etching has optimum values and is determined uniquely. Generally, the residual thickness after etching is set 0.001–0.15 μm thicker than the p-type guiding layer.
Another example of known methods to realize oscillations in a single horizontal-lateral mode (only in the basic horizontal-lateral mode) through up to a high output power is the use of absorption material to be formed into the buried layer 1110 in the nitride semiconductor laser element shown in FIG. 11 so that light radiated from the active layer 1105 can be absorbed by the absorption material. This is disclosed in, for example, “Japanese Journal of Applied Physics, Vol. 41, pp. 1829–1833, 2002” by Tsuyoshi Tojyo et al. In the nitride semiconductor laser element described in this non-patent document 3, the buried layer 1110 in the nitride semiconductor laser element shown in FIG. 11 is an adsorption layer made of oxidized silicon (SiO2), or is an adsorption layer having a two-layer structure of several tens of nanometers consisting of an oxidized silicon (SiO2) layer and a silicon (Si) layer.
FIG. 17 is a schematic view of the basic configuration of a conventional optical data recording/reproducing device 1708. This device 1708 is composed of a semiconductor laser element 1701, a collimating lens 1702, a beam splitter 1703, an objective lens 1704, a light detecting system 1706 to detect light, and a molded prism 1707. The semiconductor laser element used here is a conventional one.
In the recording operation, a laser beam emitted from the semiconductor laser element is converted into parallel light or nearly parallel light by the collimating lens, collected by the objective lens through the beam splitter, and then radiated to the data recording surface of a light disk 1705. Bit data is written onto the data recording surface of the light disk 1705 with the projections and depressions, and magnetic modulation or refractive index modulation. In the erasing operation, through the same passage as the recording operation, a laser beam is radiated to the data recording surface of the light disk to erase the recorded bit data. In the reproducing operation, through the same passage as the recording and erasing operations, a laser beam emitted from the semiconductor laser element is radiated to the data recording surface of the light disk on which bit data is written with the projections and depressions, and magnetic modulation or refractive index modulation. Then, the laser beam is reflected by the reflecting surface of the light disk and transmitted through the objective lens and the beam splitter, before entry into the light detecting system. The light detecting system converts the detected light into an electrical signal and reads the recorded data.
Ellipticity of laser beams emitted from conventional semiconductor laser elements is so high that a correction (to make ellipticity close to 1) has been needed with a molded prism before radiation to the light disk.